feat(embedding): add MiniLM support and env var configuration

[mars167]

Summary

Add npm auto-publish workflow and MiniLM embedding model support.

Changes

Code Changes

• src/core/embedding/semantic.ts: Add GIT_AI_EMBEDDING_MODEL env var support
• docs/embedding.md: New comprehensive embedding documentation

CI/CD Changes

• .github/workflows/npm-publish.yml: New GitHub Actions workflow for automatic npm publishing

Features

npm Auto-Publish Workflow

• Automatic: Publishes when version tag is pushed (e.g., v2.2.0)
• Manual: Supports workflow_dispatch for manual triggering
• Pipeline: Test → Publish → Version Bump
• Auth: Supports NPM_TOKEN and OIDC

MiniLM Support

• Lightweight model (86MB, 384 dimensions)
• Environment variable configuration
• Auto-dimension detection

Motivation

• Enable automated npm publishing via GitHub Actions
• Support lightweight MiniLM model for faster local development
• Provide clear documentation for embedding model setup

Testing

• ✅ MiniLM model downloaded and verified
• ✅ Semantic search tested
• ✅ Workflow YAML validated

Breaking Changes

None - backward compatible